J. Phys. Chem. B 2003, 107, 6739-6742
6739
Direct Synthesis of Gallium Nitride Nanowires Coated with Boron Carbonitride Layers
Hee Won Seo, Seung Yong Bae, and Jeunghee Park*
Department of Chemistry, Korea UniVersity, Jochiwon 339-700, South Korea
Hyunik Yang
College of Engineering Science, Hanyang UniVersity, Ansan 425-791, South Korea
Bongsoo Kim
Department of Chemistry, Korea AdVanced Institute of Science and Technology, Daejeon 305-701, South Korea
ReceiVed: January 1, 2003; In Final Form: April 30, 2003
Gallium nitride nanowires coated with boron carbonitride layers were directly synthesized by thermal chemical
deposition using the reaction of gallium, gallium oxide, boron oxide, and carbon nanotubes under an ammonia
atmosphere. The average diameter is 30 nm, and the length is up to 1 mm. Less than 20 graphitic sheets coat
the single-crystalline wurtzite-structured gallium nitride nanowires. The graphitic outerlayers are composed
of boron, carbon, and nitrogen atoms with a ratio of about 1:2:1. The photoluminescence exhibits a broad
band in the energy range 2.1-3.6 eV, suggesting a contribution of the emission from the graphitic boron
carbonitride outerlayers.
1. Introduction
Since the discovery of carbon nanotubes (CNTs), much
technological and scientific excitement has been raised by the
discovery of various forms of nanostructures.1-4 There has
recently been a great deal of interest in the preparation and
characterization of gallium nitride (GaN) nanowires and their
nanocomposites because the GaN material is one of the
important semiconductors in many applications including UV
or blue emitters and detectors, high-speed field-effect transistors,
and high-temperature microelectronic devices.5-7 A number of
research groups succeeded in synthesizing GaN nanowires using
various methods such as carbon-nanotube-confined reaction,8
arc discharge,9 laser ablation,10 sublimation,11 pyrolysis,12 and
chemical vapor deposition (CVD).13-18 The synthesis of a
GaN-carbon nanocomposite was first demonstrated by the arc
discharge method, producing GaN nanorods coated with carbon
multilayers.8 Han and Zettl reported the synthesis of GaN
nanorods coated with graphitic layers by the deposition of carbon
layers on preproduced GaN nanorods.19 They also synthesized
GaN nanorods coated with boron nitride (BN) layers.20 The
mixture of Ga, gallium oxide (Ga2O3), and amorphous boron
powder reacts with flowing NH3. The synthesis of not only GaN
nanowires but also many other semiconductor (e.g., Si, SiC,
Ge) nanorods coated with graphite or BN layers was reported.21-24
In this paper, we first report on boron carbonitride (BCN)
layers coated on GaN nanowires. The bulk quantity of GaN
nanowires coated with graphitic BCN multilayers has been
synthesized directly via a thermal CVD method. The reaction
of a Ga source (Ga and ball-milled Ga2O3 mixture) with NH3
in the presence of ball-milled boron oxide (B2O3) and multiwalled CNTs produces exclusively coated GaN nanowires on
* To whom correspondence should be addressed. E-mail: parkjh@
korea.ac.kr.
a large area of iron (Fe) nanoparticle deposited alumina
substrates. The structural and optical properties of the coated
GaN nanowires have been investigated using scanning electron
microscopy (SEM), transmission electron microscopy (TEM),
electron diffraction (ED), electron energy loss spectroscopy
(EELS), X-ray diffraction (XRD), Raman spectroscopy, and
photoluminescence (PL).
2. Experimental Section
Ga2O3 (99.9999%, Mateck) and B2O3 (99.9%, Aldrich) were
separately ball-milled for 20 h, using a mechanical ball mill
system (Spex 8000M). The multiwalled CNTs were grown using
a thermal CVD of acetylene at 900 °C. The diameter is in the
range of 20-60 nm, and the length is about 30 µm. About
0.5 g of Ga (99.999%, Aldrich), ball-milled Ga2O3, ball-milled
B2O3, and CNTs mixed with the same volume ratio were placed
inside a 2.5 cm diameter and 80 cm long quartz tube reactor.
Alumina substrate (1 cm × 2 cm) was dipped into 0.1 M FeCl2‚
4H2O (99%, Aldrich) ethanol solution prepared in an argon (Ar)filled glovebox and dried by Ar. The substrate was transferred
at a distance of 5-10 cm from the Ga source. The temperature
of the source was set at 1100 °C, and that of the substrate was
approximately 950 °C. While the temperature was raised, Ar
flowed at a rate of 500 sccm using a mass-flow controller. NH3
(99.999%, Solkatronic) was introduced at a rate of 200 sccm.
The growth time was 1 h. SEM (Hitachi S-4300), TEM (JEOL
JEM-2010, 200 kV), ED, and EELS (GATAN GIF-2000)
coupled with TEM (Philips CM200, 200 kV) were employed
to examine the morphology and structure of the products. XRD
(Philips X’PERT MPD) patterns and Raman spectra (Renishaw
1000) excited by the 514.5 nm line of an argon ion laser were
measured. The PL measurement was conducted using a 325 nm
helium-cadmium (He-Cd) laser as the excitation source.
10.1021/jp0340055 CCC: $25.00 © 2003 American Chemical Society
Published on Web 06/18/2003
6740 J. Phys. Chem. B, Vol. 107, No. 28, 2003
Seo et al.
Figure 1. SEM micrographs showing (a) the wool-like GaN nanowires
grown on a large area of the alumina substrate and (b) the high-density
pure GaN nanowires.
Figure 2. (a) TEM image showing the general morphology of the
GaN nanowires in which all of them are straight and coated with the
layers. The average diameter is 30 nm. (b) An enlarged view revealing
the homogeneous coating of the GaN nanowires.
Figure 3. (a) HRTEM image of a wurtzite GaN nanowire grown in
the [001] growth direction. The SAED pattern shows the [002] direction
of the graphitic layers indicated by an arrow (inset). (b) The fringes of
the crystalline graphitic layers are separated by 0.33-0.34 nm. (c)
HRTEM image and SAED pattern of another GaN nanowire whose
growth direction is [010]. The arrow indicates the obvious [002]
direction of the graphitic layers (inset). (d) Atomic-resolved view
revealing the nearly defect free GaN (001) plane and low degree of
crystalline perfection of the graphitic outerlayers.
3. Results and Discussion
Wool-like product with a light gray color was homogeneously
deposited on a large area of the alumina substrates with a visible
thickness. Figure 1a shows a low-magnification SEM image of
such products. The high-density nanowires are grown with a
length up to 1 mm. A magnified view reveals the high purity
of GaN nanowires without any nanoparticles (Figure 1b).
TEM images show a typical morphology of the GaN
nanowires in which thin layers coat the straight nanowires along
the entire length (Figure 2a). The diameter is 10-50 nm with
an average value of 30 nm. The thickness of the outerlayer is
less than 5 nm (Figure 2b). The nanowires are rotated along
the wire axis, showing that the diameter is not much changed.
The cylindrical nanowires are thus uniformly coated with the
layers.
The high-resolution TEM (HRTEM) images reveal the
detailed features for an individual GaN nanowire. Figure 3a
shows that the highly crystalline GaN nanowire is coated with
∼15 sheets of the graphitic layers. The inset is the corresponding
selected-area ED (SAED) pattern, which is consistent with that
of the wurtzite GaN structure recorded along the 〈210〉 zone
axis. The growth direction is [001]. It also shows the [002]
direction of the graphitic layers indicated by the arrow. The
pattern is essentially identical over the entire nanowire, confirming the single-crystalline wurtzite structure. The fringes of the
crystalline graphitic layers are separated by 0.33-0.34 nm
(Figure 3b). In the nanowire part, the distance between
neighboring (001) planes is 0.52 nm, which is approximately
the same as that of the bulk wurtzite GaN crystal.25 A few
stacking faults exist along the stacking of the (001) planes.
Figure 3c corresponds to the HRTEM image of another GaN
nanowire coated with about seven graphitic sheets. The growth
direction of the GaN nanowire is [010]. The ED pattern clearly
shows the [002] direction of the graphitic layers indicated by
Figure 4. (a) EELS data for the coated GaN nanowire. The graphitic
outerlayers show the K shell ionization edges of boron (B-K), nitrogen
(N-K), and carbon (C-K) at 188, 280, and 400 eV, respectively. The
atomic ratios B/C and N/C are 0.46 and 0.73, respectively. The nanowire
part shows only N-K edges. (b) Element map of carbon obtained using
the inelastic electrons corresponding to the energy loss of K shell edges
of carbon.
the arrow (inset). Figure 3d is an enlarged image for part of
Figure 3c, showing that the graphitic sheets are waved over a
short range and thus are more defective than those of the
nanowires shown in Figure 3b. The growth direction of the
nanowires is not uniform, and the degree of the crystalline
perfection of the graphitic layers also varies, but all GaN
nanowires are nevertheless coated with the graphitic layers.
The EELS data of the GaN nanowire are shown in Figure
4a. They are taken from two regions, the GaN nanowire and
the graphitic outerlayer parts. The spot size of the electron probe
is about 2 nm. The graphitic outerlayer part shows three distinct
absorption features starting at 188, 280, and 400 eV, corresponding to the known K-shell ionization edges for boron (BK), carbon (C-K), and nitrogen (N-K), respectively. A detailed
inspection of the near-edge fine structure confirms the sp2
hybridization state for boron, distinguished by the sharply
defined π* features. For the C-K edge, a defining π* peak at
Synthesis of GaN Nanowires Coated with BCN Layers
Figure 5. (a) XRD pattern and (b) Raman scattering spectrum of the
GaN nanowires coated with BCN layers. The excitation wavelength is
514.5 nm from the argon ion laser.
288 eV corresponds to the graphitic carbons. For the N-K edge,
the π* peak at around 405 eV is not completely split with the
σ* peak, which is probably due to the contribution of the GaN
nanowire. The relative atomic ratios B/C and N/C are 0.46 and
0.73, respectively. The ratio B/C varies in the range 0.4-0.5,
while the ratio N/C varies in the range 0.4-0.7, depending on
the nanowires. In the GaN nanowire part, only the N-K edge
appears. The data suggest that the GaN nanowires would be
coated with the graphitic BCN layers whose composition ratio
is approximately B/C/N ) 1/2/1. The EELS (or energy-filtered
TEM) imaging of the nanowires has been obtained using the
inelastic electrons corresponding to C-K edges. Figure 4b shows
the C elemental map for the GaN nanowires coated with BCN
layers. The brighter points represent a higher concentration of
the element. This element mapping confirms that all of the
nanowires are homogeneously coated with the C element.
We irradiated the electron beam of the microscope onto the
graphitic outerlayers for about 30 min, and found that the
outerlayers were not removed but the nanowires were collapsed
instead. Han and Zettl reported the removal of the carbon layers
by irradiation for several minutes. The BCN composition
probably enhances the susceptibility to the electron beam
damage.19 The BCN layers would act as better protecting layers
than the carbon layers for the GaN nanowires.
The XRD pattern shows the peaks of the wurtzite GaN crystal
for the nanowires detached from the substrate (Figure 5a). The
peak from the other crystalline phase is negligible. The Miller
indices are indicated on each peak. A careful examination of
the peak position reveals that the (100), (002), and (101) peaks
shift to higher angles from those of the commercial GaN
powders (99.999%, Aldrich) by 0.1-0.15°. A similar value of
the angle shift was also observed from the uncoated GaN
nanowires that were previously reported by our group.17 Since
the nanowires are randomly oriented on the substrate, the XRD
can reflect mostly the separation of the lattice planes parallel
to the growth direction. The reduction of lattice constants
suggests that the nanowires would experience compressive
strains toward the radial direction and tensile strains induced
along the growth direction.
The Raman scattering spectrum of the GaN nanowires is
displayed in Figure 5b. The first-order phonon frequencies of
A1(TO), E2(high), and A1(LO) are known to be 533, 570, and
740 cm-1, respectively.26,27 The peaks at 532, 567, and 710 cm-1
can be assigned to the A1(TO), E2(high), and A1(LO) modes,
respectively. The shift of the E2(high) peak to the lower
frequency region from that of the bulk would be correlated with
the tensile strain inside the nanowires, which is consistent with
the results of the uncoated GaN nanowires.17,28 The first-order
Raman peak of the BCN film or multiwalled BCN nanotubes
J. Phys. Chem. B, Vol. 107, No. 28, 2003 6741
Figure 6. PL spectrum of the BCN layer coated GaN nanowires
measured at room temperature (300 K). The excitation laser is the 325
nm line of a He-Cd laser.
usually consists of two bands at ∼1580 cm-1 (G band) and
∼1350 cm-1 (D band).29,30 The G band originates from the
Raman active E2g mode due to in-plane atomic displacements.
The origin of the D band has been explained as a disorderinduced feature due to the finite particle size effect or lattice
distortion. Two bands at ∼1580 and ∼1350 cm-1 can be the G
and D bands, respectively. The appearance of two bands
provides another indication for the coated BCN multilayers.
The PL spectrum obtained from the BCN layer coated GaN
nanowires at 300 K shows a broad band in the energy range of
2.1-3.6 eV with the peak at 3.36 eV (Figure 6). The wellknown yellow band centered at 2.2-2.3 eV is not noticeable.31
The PL of uncoated GaN nanowires was observed in the broader
range 2.9-3.6 eV with the peak at lower energy compared to
that of the epilayer.17 The results were interpreted by the change
of the band gap energy due to the strains predicted from the
data of XRD and the Raman spectrum. The various strengths
of the strain would shift the band gap in the broader range. The
tensile strains are dominant, resulting in the reduction of the
band gap energy. The PL properties of the present GaN
nanowires are also probably related to such a band gap change.
However, the weak emission in the lower energy range
2.1-2.9 eV was not found from the uncoated GaN nanowires.
This may be related to defects such as atomic vacancies, existing
possibly at the interface of the nanowires with the BCN layers.
Another possible origin is the graphitic BCN layers. The PL
from the layered BC2N compounds shows a peak at 2.1 eV at
room temperature.32,33 The BCN films showed a shift of the
PL peak from 2.80 to 3.4 eV with increasing B content.34 The
peak of the BCN film appears in the broad range 2.0-3.6 eV.
Recently, the blue-violet PL of the highly aligned BCN
nanofibers in the range of 2.6-3.2 eV was reported.35 When
the B content increases, the peak centers shift to higher energy.
Therefore, the emission from the BCN layers would add to the
PL of the GaN nanowires.
It is known that when a Ga and Ga2O3 mixture is used,
Ga2O vapor is generated as follows: 4Ga(l) + Ga2O3(s) f
3Ga2O(g).36 The ball-milled Ga2O3 powders are nearly amorphous, so they can produce a sufficient vapor pressure of Ga2O
to grow the GaN nanowires on a large scale. The reaction
involved in the growth of the nanowires is probably Ga2O(g)
+ 2NH3(g) f 2GaN(s) + H2O(g) + 2H2(g).37 Since the growth
of nanowires is negligible without the catalyst, it would follow
a typical vapor-liquid-solid (VLS) growth mechanism.3 The
Ga2O vapor deposits on the Fe catalytic nanoparticles, forming
a miscible liquid alloy. The continuous dissolving of the Ga
and N sources would lead to the precipitation of GaN. Our group
synthesized the GaN nanobelts using B2O3 as a catalyst under
NH3 flow.38,39 No BN outerlayers exist on those nanobelts. In
this experiment the multiwalled CNTs provide the C source to
react with B2O3 and NH3, depositing the multiwalled BCN
6742 J. Phys. Chem. B, Vol. 107, No. 28, 2003
outerlayers on the GaN nanowires. The reaction may be 4C(g)
+ B2O3(g) + 2NH3(g) f 2BC2N(s) + 3H2O(g). The GaN
nanowire can play as a template for the growth of the graphitic
BCN layers. The result suggests that the formation of BCN
layers would be more efficient than that of BN layers under
our growth conditions.
4. Conclusions
The graphitic BCN layer coated GaN nanowires were
synthesized in large quantity by the thermal CVD method. The
mixture of Ga, ball-milled Ga2O3, and B2O3 powders, and
multiwalled CNTs reacts under NH3 atmosphere. The temperature of the source was maintained as 1100 °C, and the
nanowires were grown on the Fe nanoparticle deposited alumina
substrates at 950 °C. The light-gray-colored wool-like nanowires
were produced with a visible thickness. The average diameter
is 30 nm, and the length is up to 1 mm.
The TEM, ED, and EELS analyses show that less than 20
graphitic sheets coat the single-crystalline wurtzite-structured
GaN nanowires whose growth direction is not uniform. For the
graphitic outerlayers, the EELS estimates the atomic ratio B/C/N
to be about 1/2/1. XRD and Raman spectroscopy confirm the
high crystallinity of the wurtzite GaN crystals and the existence
of graphitic BCN layers. They also suggests that the lattice
constants of the nanowires would be changed via the strains.
The PL exhibits a broad band in the energy range 2.1-3.6 eV,
excited by the 325 nm line of a He-Cd laser. The results are
possibly explained in terms of the band gap decrease due to
the dominant tensile strains. The emission from the BCN
outerlayers may contribute to the PL in the lower energy range
2.1-2.9 eV. The GaN nanowires would grow following the VLS
mechanism. They probably act as template for the formation
of the graphitic BCN outerlayers.
Acknowledgment. The National R&D Project for Nano
Science and Technology (KISTEP) supported the present work.
SEM and XRD analyses were performed at the Basic Science
Research Center in Seoul. We thank Dr. S. Y. Lee and Dr. J.
C. Park (Hynix) for the EELS data.
References and Notes
(1)
(2)
1335.
(3)
(4)
Iijima, S. Nature 1991, 354, 56.
Murray, C. B.; Kagan, C. R.; Bawendi, M. G. Science 1995, 270,
Hu, J.; Odom, T. W.; Lieber, C. M. Acc. Chem. Res. 1999, 32, 435.
Pan, Z. W.; Dai, Z. R.; Wang, Z. L. Science 2001, 291, 1947.
Seo et al.
(5) Mohammad, S. N.; Morkoç, H. Prog. Quantum Electron. 1996,
20, 361.
(6) Ponce, F. A.; Bour, D. P. Nature (London) 1997, 386, 351.
(7) Nakamura, S. Science 1998, 281, 956.
(8) Han, W.; Fan, S.; Li, Q.; Hu, Y. Science 1997, 277, 1287.
(9) Han, W.; Redlich, P.; Ernst, F.; Rühle, M. Appl. Phys. Lett. 2000,
76, 652.
(10) Duan, X.; Lieber, C. M. J. Am. Chem. Soc. 2000, 122, 188.
(11) Li, J. Y.; Chen, X. L.; Qiao, Z. Y.; Cao, Y. G.; Lan, Y. C. J. Cryst.
Growth 2000, 213, 408.
(12) Han, W.-Q.; Zettl, A. Appl. Phys. Lett. 2002, 80, 303.
(13) Chen, X.; Li, J.; Cao, Y.; Lan, Y.; Li, H.; He, M.; Wang, C.; Zhang,
Z.; Qiao, Z. AdV. Mater. 2000, 12, 1432.
(14) Chen, C.-C.; Yeh, C.-H.; Chen, C.-H.; Yu, M.-Y.; Liu, H.-L.; Wu,
J.-J.; Chen, K.-H.; Chen, L.-C.; Peng, J.-Y.; Chen, Y.-F. J. Am. Chem. Soc.
2001, 123, 2791.
(15) Zhang, J.; Zhang, L. D.; Wang, X. F.; Liang, C. H.; Peng, X. S.;
Wang, Y. W. J. Chem. Phys. 2001, 115, 5714.
(16) Lee, M. W.; Twu, H. Z.; Chen, C.-C.; Chen, C.-H. Appl. Phys.
Lett. 2001, 79, 3693.
(17) Seo, H. W.; Bae, S. Y.; Park, J.; Yang, H.; Park, K. S.; Kim, S. J.
Chem. Phys. 2002, 116, 9492.
(18) Kim, H.-M.; Kim, D. S.; Park, Y. S.; Kim, D. Y.; Kang, T. W.;
Chung, K. S. AdV. Mater. 2002, 14, 991.
(19) Han, W.; Zettl, A. AdV. Mater. 2002, 14, 1560.
(20) Han, W.; Zettl, A. Appl. Phys. Lett. 2002, 81, 5051.
(21) Zhang, Y.; Suenaga, K.; Colliex, C.; Iijima, S. Science 1998, 281,
973.
(22) Tang, C.; Bando, Y.; Sato, T.; Kurashima, K. AdV. Mater. 2002,
14, 1046.
(23) Sun, X.-H.; Li, C.-P.; Wong, W.-K.; Wong, N.-B.; Lee, C.-S.; Lee,
S.-T.; Teo, B.-K. J. Am. Chem. Soc. 2002, 124, 14464.
(24) Wu, Y.; Yang, P. AdV. Mater. 2001, 13. 520.
(25) Neumayer, D. A.; Ekerdt, J. G. Chem. Mater. 1996, 8, 9.
(26) Azuhata, T.; Sota, T.; Suzuki, K.; Nakamura, S. J. Phys.: Condens.
Matter 1995, 7, L129.
(27) Davydov, V. Yu.; Kitaev, Yu. E.; Goncharuk, I. N.; Smirnov, A.
N.; Graul, J.; Semchinova, O.; Uffmann, D.; Smirnov, M. B.; Mirgorodsky,
A. P.; Evarestov, R. A. Phys. ReV. B 1998, 58, 12899.
(28) Klose, M.; Wieser, N.; Rohr, G. C.; Dassow, R.; Scholz, F.; Off,
J. J. Cryst. Growth 1998, 189/190, 634.
(29) Yap, Y. K.; Yoshimura, M.; Mori, Y.; Sasaki, T. Appl. Phys. Lett.
2002, 80, 2559.
(30) Zhi, C. Y.; Bai, X. D.; Wang, E. G. Appl. Phys. Lett. 2002, 80, 3590.
(31) Chen, H. M.; Chen, Y. F.; Lee, M. C.; Feng, M. S. Phys. ReV. B
1997, 56, 6942.
(32) Watanabe, M. O.; Itoh, S.; Sasaki, T.; Mizushima, K. Phys. ReV.
Lett. 1996, 77, 187.
(33) Chen, Y.; Barnard, J. C.; Palmer, R. E.; Watanabe, M. O.; Sasaki,
T. Phys. ReV. Lett. 1999, 83, 2406.
(34) Yu, J.; Wang, E. G.; Ahn, J.; Yoon, S. F.; Zhang, Q.; Cui, J.; Yu,
M. B. J. Appl. Phys. 2000, 87, 4022.
(35) Bai, X. D.; Wang, E. G.; Yu, J.; Yang, H. Appl. Phys. Lett. 2000,
77, 67.
(36) Frosch, C. J.; Thurmond, C. D. J. Phys. Chem. 1962, 66, 877.
(37) Balkaş, C.; Davis, R. F. J. Am. Ceram. Soc. 1996, 79, 2309.
(38) Bae, S. Y.; Seo, H. W.; Park, J.; Yang, H.; Park, J. C.; Lee, S. Y.
Appl. Phys. Lett. 2002, 81, 126.
(39) Bae, S. Y.; Seo, H. W.; Park, J.; Yang, H.; Song, S. A. Chem.
Phys. Lett. 2002, 365, 525.